1. Field of the Invention
The present invention relates to an atomic oscillator suitable for use as a standard source in broadcasting, a clock source in a digital synchronous network, etc.
2. Description of the Related Art
FIG. 6 of the accompanying drawings is a block diagram schematically showing a conventional rubidium atomic oscillator (hereinafter simply called "atomic oscillator"). The atomic oscillator 101 is divided into chiefly three blocks: a high frequency (HF) block 102 as a first block, an optical microwave unit (OMU) 103 as a second block, and a low frequency (LF) block 104 as a third block.
The HF block 102 not only generates an output frequency signal to be supplied to outside as the output of the atomic oscillator 101, but also produces from the output frequency signal a signal from which a microwave {rubidium resonant (transitional) frequency signal of approximately 6.8346 GHz} to be produced in the OMU 103 originates. For this purpose, in an example, the HF block 102 essentially includes a voltage-controlled crystal oscillator (VCXO) 120 as a standard oscillator, a first LC tank circuit 121, a phase modulator circuit 122, a second LC tank (resonator) circuit 123, an amplitude modulator circuit 124, a matching circuit 125, and a frequency synthesizer 126.
The VCXO 120 generates an output frequency signal (e.g., 10 MHz) to be supplied to outside as the output of the atomic oscillator. In an example, each of the first and second LC tank (resonator) circuits (hereinafter simply called "tank circuit") 121, 123 includes a non-illustrated transistor amplifier, and a tank filter composed of a coil (L) and a capacitor (C) in parallel; the individual tank circuit 121, 123 varies a bias of the transistor amplifier to distort an input frequency signal and then samples from the resultant signal a multiplied-frequency component, thus multiplying the input frequency signal by a predetermined natural number (3 for example). Firstly in the first tank circuit 121, the output frequency signal (10 MHz for example) of the VCXO 120 is multiplied by a natural number (3, for example); this is, 10 MHz.times.3=30 MHz. Then in the second tank circuit 123, the resultant output frequency signal of 30 MHz is multiplied by the same natural number; this is, 30 MHz.times.3=90 MHz.
In the meantime the phase modulator circuit 122 modulates the output (30 MHz) of the first tank circuit 121 in phase (or in frequency) using the output (phase modulation signal fm of 155 Hz, for example) of a below-described low frequency oscillator 141 of the LF block 104. The amplitude modulator circuit 124 modulates the output (90 MHz) of the second tank circuit 123 by the output (approximately 5.3101 HMz) of the frequency synthesizer 126.
The frequency synthesizer 126, as is well known in the art, is a kind of phase-locked loop (PLL) circuit equipped with a non-illustrated voltage-controlled oscillator (VCO) for generating a frequency signal for the above-mentioned amplitude modulation (amplitude modulation signal of approximately 5.3101 MHz). The output of the VCO is compared in phase with the output of the VCXO 120, and the output frequency (amplitude modulation frequency) of the VCO is controlled in such a manner that the output of the VCO is synchronized in phase with that of the VCXO 120. The PLL-type frequency synthesizer 126 is also equipped with a variable frequency divider (programmable frequency divider) so that fine adjustments of the above-mentioned amplitude modulation frequency can be made in accordance with frequency setting information given from an external source.
Further, the matching circuit 125 takes an impedance matching between the HF block 102 and the OMU 103 so that the signal (90 MHz) amplitude-modulated in the amplitude modulator circuit 124 is input to the OMU 103 via the matching circuit 125 as a high frequency signal from which the above-mentioned resonant frequency signal originates.
Then the OMU 103, which is a box-shaped atomic resonator, detects and outputs a signal (atomic resonance signal) when a rubidium atom in the resonator box is resonated (transited) as a microwave to be a resonant (transitional) signal of a rubidium atom occurs in the resonator box. For this purpose, as shown in FIG. 6, the OMU 103 includes a rubidium lamp 131, a resonance cell 133 in which a rubidium atom is charged (loaded), a cavity resonator 132 equipped with a varactor diode 134 and a photo diode (PD), and a pre-amplifier (PA) 136. The resonator box of the OMU 103 is treated with magnetic shielding 130 so that resonance of rubidium atom is prevented from being influenced by a possible magnetic field due to a peripheral circuit.
The above-mentioned rubidium lamp 131 is a lamp for emitting light (high frequency discharge) by exciting a coil 1312 with the output of an exciter 1311, thereby irradiating the light to a resonance cell 133 in the cavity resonator 132.
The varactor diode (high-natural-number multiplier) 134 produces in the cavity resonator 132 a microwave (resonant frequency signal) having an amplitude-modulated component .+-.5.3101 MHz at either side about 90.times.76=6.840 GHz by multiplying the output (phase-and amplitude-modulated high frequency signal of 90 MHz) by a high natural number (76 for example) of the matching circuit 125; the lower part of this microwave frequency (6.840 GHz-5.3101 MHz=6.8346 GHz) is a value approximating to the resonant frequency of rubidium atom. The cavity resonator 132 is tuned to approximately 6.8 GHz (designed in such a manner that resonance will occur under a microwave of approximately 6.8 GHz).
Since the requested output frequency of the VCXO 120 is oddlessly 10 MHz, simply multiplying such output frequency 10 MHz using the first and second tank circuits 121, 123 and the varactor diode 134 does not suffice to produce the resonant frequency of rubidium atom (approximately 6.8346 GHz). Consequently in the conventional rubidium atomic oscillator 101, the input signal to the OMU 103 is modulated in amplitude whereupon the amplitude-modulated component of the resultant input signal is multiplied.
The photo diode 135 receives the light emitted from the rubidium lamp 131 and traveling through the resonance cell 133 and outputs an electrical signal in accordance with the quantity of the received light as an atomic resonance signal. Assuming that a microwave frequency produced by the varactor diode 134 coincides with the resonant frequency (approximately 6.8346 GHz) of rubidium atom, an atomic resonance will occur so that the light from the rubidium lamp 131 will be absorbed in part by the rubidium atom in the resonance cell 133, resulting in reduced quantity of the received light.
Accordingly an electrical signal (atomic resonance signal) to be output from the photo diode 135 has information about a difference (error frequency) between the microwave frequency, which is produced in the cavity resonator 132, and the resonant frequency of rubidium atom (approximately 6.8346 GHz). The pre-amplifier 136 serves to amplify the output of the photo diode 135 to an appropriate level in advance.
The LF block 104 serves as a servo circuit which detects the above-mentioned frequency-error information (error signal) from the atomic resonance signal output from the OMU 103 and which then controls the output frequency of the VCXO 120 in such a manner that the detected error signal is minimal (ideally 0). In an example, the LF block 104 is composed of a low-frequency voltage control oscillator (VCO) 141, an amplifier 142, a synchronous detector 143, and an integrator 144.
The low-frequency VCO 141 generates a phase modulation signal fm for the phase modulator circuit 122 and, in the meantime, the amplifier 142 amplifies an atomic resonance signal from the OMU 103 up to an appropriate level. And the synchronous detector 143 takes a synchronous detection on the atomic resonance signal, which is amplified by the amplifier 142, with the output (phase modulation signal fm) of the low-frequency VCO 141 to thereby detect an error signal.
The integrator 144 integrates the error signal, which is detected by the synchronous detector 143, to convert the error signal into a d. c. voltage value; this d. c. voltage is to be supplied (applied) to the VCXO 120 as a control voltage to control (correct) the output frequency thereof.
The operation of the conventional atomic oscillator 101 will now be described.
First of all, the output (10 MHz) of the VCXO 120 is input, as the output of the rubidium atomic oscillator 101, to both the first tank circuit 121 of the HF block 102 and the frequency synthesizer 126. The frequency synthesizer 126 then produces a signal of approximately 5.3101 MHz, which is synchronized in phase with the output frequency of the VCXO 120, as an amplitude modulation signal for the amplitude modulator circuit 124.
Meanwhile, the first tank circuit 121 multiplies the input signal (10 MHz) by 3 and outputs the resultant input signal (10 MHz.times.3=30 MHz). The output of the first tank circuit 121 is input to the phase modulator circuit 122 where the output of the first tank circuit 121 is modulated in phase by the output (phase modulation signal fm) of the low-frequency VCO 141 in the LF block 104. Then the output of the phase modulator circuit 122 is further multiplied by 3 (30 MHz.times.3=90 MHz) in the second tank circuit 123, whereupon the resultant output of the phase modulator circuit 122 is modulated in amplitude by the output (amplitude modulation signal) of the frequency synthesizer 126 in the amplitude modulator circuit 124 and is then input to the OMU 103 via the matching circuit 125.
Subsequently, in the OMU 103, the input signal from the matching circuit 125 is multiplied by 76 (high natural number) by the action of the varactor diode 134 in the cavity resonator 132 so that a microwave of approximately 6.840 GHz occurs in the cavity resonator 132, causing atomic resonance in the cavity resonator 132.
At that time, if the lower part of the amplitude-modulated component about the microwave frequency occurred by the action of the varactor diode 134 coincides with the resonant frequency of rubidium atom (6.8346 GHz), atomic resonance will occur so that the light from the rubidium lamp 131 will be absorbed by the rubidium atom in the resonance cell 133, thus resulting in a drastically reduced quantity of light to be detected by the photo diode 135 (see a point A in FIG. 7).
Otherwise if the lower part of the amplitude-modulated component is off even by little the resonant frequency of rubidium atom (6.8346 GHz) the quantity of light to be detected by the photo diode 135 will drastically increase (see points B and C in FIG. 7) because Q value is very small (500 Hz for example) compared to the resonant frequency, as shown in FIG. 7.
Thus the quantity of light (electrical signal) detected by the photo diode 135 is input to the LF block 104 via the pre-amplifier 136. At that time, the output of the pre-amplifier 136, which has been obtained by modulating in phase the microwave generated in the cavity resonator 132, is a signal (atomic resonance signal) inverted in phase by 180.degree. about the resonant frequency near 6.8346 GHz as schematically shown in (a) of FIG. 8. A, B and C in (a) of FIG. 8 represent the respective output waveforms of the pre-amplifier 136 with respect to the microwave frequencies at points A, B and C in FIG. 7.
The LF block 104 converts the output (atomic resonance signal) of the pre-amplifier 136 to an appropriate level at the amplifier 142 and then takes a synchronous detection at the synchronous detector 143 by the phase modulation frequency fm (see (b) of FIG. 8 for example) from the low frequency oscillator 141 to produce an error signal (see (c) of FIG. 8).
When this error signal is converted into a d. c. voltage by integrating the error signal by the integrator 144, various voltage values of 0 (zero), - (negative) and + (positive) will occur at point A (there is no difference or error between the microwave frequency and the resonant frequency), point B (the microwave frequency is higher than the resonant frequency) and point C (the microwave frequency is lower than the resonant frequency), respectively; the occurred voltage is to be applied to the VCXO 120 as a control voltage therefor.
The output frequency of the VCXO 120 is thereby controlled (corrected) in such a manner that a microwave frequency (the lower part of the amplitude-modulated component) to be produced in the cavity resonator 132 coincides with the resonant frequency of rubidium atom (6.8346 GHz); this is, the output frequency of the VCXO 120 is stabilized with the degree of stability of the resonant frequency of rubidium atom. With this conventional rubidium atomic oscillator 101, it is possible to produce a very high-stability output frequency.
In the above-mentioned example, adopting a natural number "3" as a multiplier of each of the first and second circuits 121, 123 and a two-digit natural number "76" as a multiplier natural number of the varactor diode 134 (namely, a signal of 90 MHz obtained by multiplying the output of the VCXO 120 by 9 (3.times.3) is modulated in amplitude, and the resulting frequency is then multiplied by 76), the resonant frequency of rubidium atom is produced. Alternatively, even if the respective multipliers of the first and second tank circuits 121, 123 are "2" and "3" and the multiplier of the varactor diode 134 is "144" (namely, a signal of 60 MHz obtained by multiplying the output of VCXO 120 by 6 (2.times.3) is modulated in amplitude, and the resulting frequency is then multiplied by 144), it is possible to produce the same resonant frequency.
Further, the conventional rubidium atomic oscillator 101 produces, as its output frequencies (i.e., the output frequencies of the VCXO 120), various output frequencies (oddless frequencies in general) of 5 MHz, for example, other than that of 10 MHz, by merely changing the respective amplifiers, natural numbers, of the first tank circuit 121, the second tank circuit 123 and the varactor diode 134 in such a manner that a target microwave frequency to be finally produced in the cavity resonator 132 will coincide with the resonant frequency of rubidium atom, irrespective of the figure of the output frequency.
However, with the foregoing atomic oscillator 101, since the tank circuits 121, 123 for extracting a multiplied-frequency component from the input signal using a coil (L) and a capacitor (C) are used in order to multiply the output of the VCXO 120, it would be necessary to make tuning in accordance with a selected multiplier, i.e. a natural number, for every tank circuit during the manufacture of the atomic oscillator, causing a delay of term of manufacture and an increase of cost of manufacture.
Further, partly since the varactor diode 134 is a passive element, and partly since multipliers of the varactor diode 134 are "76" and "144", which are very high, strict adjustments would be essential to the matching circuit 125 of the HF block in order to produce a desired level of microwave in the cavity resonator 132. In addition, the varactor diode 134 itself is an expensive element. These facts would result in a delay of term of manufacture and an increase of cost of manufacture.
Furthermore, in the conventional rubidium atomic oscillator 101, since the output frequency of the VCXO 120 multiplied by a natural number does not coincide directly with the resonant frequency of rubidium atom (6.8346 GHz), it would require the frequency synthesizer 126 and the amplitude modulator circuit 124. Yet, in order to produce an optimum resonant frequency signal of rubidium atom, strict adjustments in modulation to the amplitude modulator circuit 124 also would be essential.
With the foregoing conventional rubidium atomic oscillator 101, partly since a complex construction is necessary to convert the output frequency of the VCXO 120 into the resonant frequency of rubidium atom, and partly since adjustments of the complex construction are not easy, it would be considerably uncompetitive in downsizing of circuits, shortening the term of manufacture, and reducing the cost of manufacture.